Typically, when the user of an oscilloscope network analyzer, spectrum analyzer, or other signal measuring device selects a spot on a curve of an input signal being measured, a trigger signal is generated corresponding to the location or position of the spot, and a dot, pip, birdie or intensity-type marker is generated at the spot on the curve coincident with the trigger signal. For example, a device that generates one such marker is described in U.S. Pat. No. 3,596,193 entitled "Sweep Oscillator with Intensity Frequency Marker", issued July 27, 1971 to inventor Duane Edmond Dunwoodie.
A problem with intensity-type markers is that, in order to generate a marker of greater intensity than that of the trace or curve of an input signal being measured, it is often necessary to briefly modify the slope of a control signal, namely, the linear ramp signal to the X-deflection axis of the CRT display of the marker device. This introduces transients, and causes errors in the measurement of the input signal at the location of the marker.
Pip and birdie markers, also briefly described in the above-mentioned patent at page 2, column 1, are often difficult to identify when the curve representing the input signal being measured includes numerous high frequency components or noise spikes, or when the sweep rate is high or the curve is moving principally in the direction of the pip or birdie. An additional disadvantage of birdie markers is that the size of these markers often depends on the signal strength of the signals combined to produce the birdie, the legibility of the birdie decreasing when such signal strength is weak. Dot markers are also difficult to identify and distinguish from the curve because they are usually placed in the curve itself and not apart from it, e.g., above or below the curve.